Electrophotographic photoreceptor and image forming apparatus using the photoreceptor

ABSTRACT

An electrophotographic photoreceptor including a photosensitive layer on an electroconductive substrate, wherein nitrate ion is present on the surface of the photosensitive layer in an amount of from 50 to 300 μg per 1 m 2  of the surface of the photosensitive layer when the nitrate ion is determined by an ion chromatographic method. Preferably a material having a fluorine atom and a carbon atom or a fatty acid metal salt such as zinc stearate is further present on the surface of the photosensitive layer such that the F/C ratio is from 0.05 to 0.5 or the Zn/C ratio is from 0.001 to 0.1. An image forming apparatus using the photoreceptor is also provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electrophotographicphotoreceptor for use in image forming apparatus such as copiers,facsimile machines, laser printers, and digital plate making machines.In addition, the present invention also relates to an image formingapparatus and process cartridge using the photoreceptor.

[0003] 2. Discussion of the Background

[0004] Electrophotographic image forming methods using a photoreceptor,which are used for copiers, facsimile machines, laser printers, directdigital plate making machines etc., are well known. The image formingmethods typically include the following processes:

[0005] (1) charging an electrophotographic photoreceptor (chargingprocess);

[0006] (2) irradiating the charged photoreceptor with imagewise light toform an electrostatic latent image thereon (light irradiating process);

[0007] (3) developing the latent image with a developer including atoner to form a toner image thereon (developing process);

[0008] (4) optionally transferring the toner image on an intermediatetransfer medium (first transfer process);

[0009] (5) transferring the toner image onto a receiving material suchas a receiving paper ((second) transfer process);

[0010] (6) fixing the toner image to fix the toner image on thereceiving material (fixing process); and

[0011] (7) cleaning the surface of the photoreceptor (cleaning process).

[0012] Currently, image forming apparatus such as copiers, facsimilemachines and laser printers tend to be for private use. Therefore, aneed exists for miniaturized image forming apparatus. In addition, imageforming apparatus having good reliability, i.e., maintenance-free imageforming apparatus are also needed.

[0013] In addition, currently image scanners and image processingapparatus such as computers are dramatically improved, and therefore itbecomes possible to prepare images having high resolution. Therefore, aneed exists for image forming apparatus which can stably produce imageshaving high resolution.

[0014] Until now, the following photoreceptors are known:

[0015] (1) photoreceptors in which a layer including an inorganicphotosensitive material such as selenium or amorphous silicon is formedon an electroconductive substrate as a photosensitive layer;

[0016] (2) photoreceptors using an organic photosensitive material;

[0017] (3) photoreceptors using a combination of an inorganicphotosensitive material and an organic photosensitive material; and

[0018] (4) photoreceptors using organic photosensitive materials.

[0019] Currently, the photoreceptors using organic photosensitivematerials are widely used because of having the following advantagesover the other photoreceptors:

[0020] (1) manufacturing costs are relatively low;

[0021] (2) it is relatively easy to design a photoreceptor having adesired property (i.e., the designing flexibility of a photoreceptor canbe increased); and

[0022] (3) hardly causing environmental pollution.

[0023] As the organic photoreceptors, the following photoreceptors areknown:

[0024] (1) photoreceptors having a photosensitive layer including aphotoconductive resin such as polyvinyl carbaozole (PVK) or the likematerial;

[0025] (2) photoreceptors having a photosensitive layer including acharge transfer complex such as a combination of polyvinyl carbaozole(PVK) and 2,4,7-trinitrofluorenone (TNF) or the like material;

[0026] (3) photoreceptors having a photosensitive layer in which apigment, such as phthalocyanine or the like, is dispersed in a binderresin; and

[0027] (4) photoreceptors having a functionally-separated photosensitivelayer including a charge generation material and a charge transportmaterial.

[0028] Among these organic photoreceptors, the photoreceptors having afunctionally-separated photosensitive layer especially attract attentionnow.

[0029] The mechanism of forming an electrostatic latent image in thefunctionally-separated photosensitive layer having a charge generationlayer and a charge transport layer formed on the charge generation layeris as follows:

[0030] (1) when the photosensitive layer is exposed to light after beingcharged, the light passes through the transparent charge transport layerand then reaches the charge generation layer;

[0031] (2) the charge generation material included in the chargegeneration layer absorbs the light and generates a charge carrier suchas electrons and positive holes;

[0032] (3) the charge carrier is injected to the charge transport layerand transported through the charge transport layer due to the electricfield formed by the charging;

[0033] (4) the charge carrier finally reaches the surface of thephotosensitive layer and neutralizes the charge thereon, resulting information of an electrostatic latent image.

[0034] For such functionally-separated photoreceptors, a combination ofa charge transport material mainly absorbing light having a wavelengthin an ultraviolet region and a charge generation material mainlyabsorbing light having a wavelength in a visible region is effective andis typically used.

[0035] However, it is well known that the functionally-separated organicphotoreceptors have a drawback of having poor mechanical and chemicaldurability. This is because low molecular weight charge transportcompounds, which have been typically developed and used as the chargetransport material, do not have film forming ability. Therefore, acombination of an inactive polymer and a low molecular weight chargetransport compound is typically used for the charge transport layer.However, such a charge transport layer is soft, and therefore has alsopoor mechanical durability. When such a photoreceptor is repeatedlycontacted to various elements such as developer, developing roller,transfer paper, cleaning brush and cleaning blade, the surface of thephotoreceptor is easily abraded due to the mechanical stress applied bythe elements.

[0036] In addition, the organic photoreceptors have another drawbacksuch that they easily react with active substances (i.e., coronadischarge induced products) such as ozone and nitrogen oxides (NOx),which are generated when charging the photoreceptors in the chargingprocess essential to electrophotography, resulting in deterioration ofcharge properties of the photoreceptors and occurrence of undesiredimages such as tailing and blurring. In particular, in order to preparea photoreceptor which can produce images having good resolution andwhich have good durability and stability, this drawback has to beremedied.

[0037] In attempting to remedy the former drawback (poor mechanicaldurability) of such an organic photoreceptor, the following techniqueshave been disclosed:

[0038] (1) a brush is used instead of a blade in the cleaning process,in which the photoreceptor is subjected to the largest mechanicalstress, to reduce the mechanical stress; and

[0039] (2) a lubricant applying device is provided in the vicinity of aphotoreceptor, which device applies a lubricant on the surface of thephotoreceptor, to decrease the abrasion of the photosensitive layer ofthe photoreceptor (this technique has been disclosed in JapaneseLaid-Open Patent Publications Nos. 6-342236, 8-202226 and 9-81001).

[0040] The abrasion can be improved by these techniques to some extent,however, the latter drawback (i.e., poor resistance to ozone and NOx)cannot be remedied. Therefore these techniques are not satisfactory.

[0041] In attempting to remedy the latter drawback of the organicphotoreceptor, the following techniques have been disclosed:

[0042] (1) Contact Charging Methods

[0043] The charging methods for charging a photoreceptor are classifiedinto two types, one of which is non-contact charging methods and theother of which is contact charging methods.

[0044] Among the non-contact charging methods, a corona dischargingmethod is well known in which a photoreceptor is charged using anelectroconductive element, such as wires and plates, which is providedapart from the surface of the photoreceptor and to which a high voltageis applied. This method has an advantage in that the surface of aphotoreceptor can be uniformly charged, and therefore the method hasbeen typically used.

[0045] On the contrary, in the contact charging methods, a photoreceptoris charged by a charging element, such as brushes, roller-shapedbrushes, rollers, blades and belts, which has an appropriateelectroconductivity and elasticity and which contacts the surface of thephotoreceptor. These methods have been disclosed in Japanese Laid-Openpatent Publications Nos. 63-149668 and 7-281503.

[0046] The contact charging methods have an advantages over thenon-contact charging methods in that the voltage applied to thephotoreceptor can be reduced and thereby the amount of generated ozone,which is considered to damage human beings and photoreceptors, can bereduced. Therefore, recently these contact charging methods have widelyspread.

[0047] (2) Short Range Charging Methods

[0048] As intermediate methods between the contact charging methods andnon-contact charging methods, short range charging methods in which a DCvoltage overlapped with a DC or AC voltage is applied to a photoreceptorusing a charging element, such as a brush, a roller-shaped brush, aroller, a blade or a belt, which has an appropriate electroconductivityand elasticity, while a narrow gap is formed between the chargingelement and the photoreceptor. These short range charging methods arepractically used recently.

[0049] When an organic photoreceptor is used, it is effective to use thecontact charging methods or short range charging methods because ofhaving the following advantages:

[0050] (1) having high charge efficiency;

[0051] (2) generation of corona-discharge-induced products such as ozoneand NOx can be reduced, resulting in prevention of occurrence ofundesired images such as blurring and tailing, thereby prolonging thelife of the photoreceptor.

[0052] With respect to the contact charging methods or short rangecharging methods, various methods have been disclosed in, for example,Japanese Laid-Open Patent Publications Nos. 56-104351, 57-178267,58-40566 and 58-150975.

[0053] However, generation of the corona-discharge-induced productscannot be perfectly avoided even when these methods are used. Therefore,high durability and stability cannot be imparted to an organicphotoreceptor only by using these methods.

[0054] In addition, in attempting to impart resistance to the chemicaland electrical stresses to an organic photoreceptor, techniques in whichan additive is added to the photosensitive layer of the photoreceptor.For example, Japanese Laid-Open Patent Publications Nos. 6-83097,7-152217 and 7-84394 have disclosed techniques in which afluorine-containing resin is included in a top layer such as aphotosensitive layer or a protective layer to control the surface energyof the layer, resulting in improvement of the chemical durability of thephotoreceptor. However, desired durability cannot be imparted to thephotoreceptor even when the addition quantity of such an additive ischanged. In addition, there is a possibility that such an additiveadversely affects the properties of the photoreceptor such as electricproperty and the like.

[0055] Because of these reasons, a need exists for anelectrophotographic photoreceptor which can produce images having goodimage qualities and which has high durability and stability.

SUMMARY OF THE INVENTION

[0056] Accordingly, an object of the present invention is to provide anelectrophotographic photoreceptor which can produce images having goodimage qualities and which has high durability and stability.

[0057] Another object of the present invention is to provide an imageforming apparatus which can stably produce images having good imagequalities without frequently changing its photoreceptor.

[0058] Briefly these objects and other objects of the present inventionas hereinafter will become more readily apparent can be attained by aphotoreceptor which includes a photosensitive layer on which nitrate ion(NO₃ ⁻) is present in an amount of from 50 to 300 μg per 1 m² of thesurface thereof, when measured by an ion chromatography method. Namely,nitrate ion detected from the surface of the photoreceptor is in theabove-mentioned range. The surface layer of the photoreceptor may be thephotosensitive layer, a protective layer or the like layer. When aprotective layer is formed as the surface layer, the layer preferablyincludes a filler and/or a charge transport material.

[0059] In addition, it is preferable that a material including afluorine atom and a carbon atom is present on the surface and when thesurface of the photoreceptor is analyzed by an X-ray photoelectronspectroscopy (XPS) method, the ratio of the number of fluorine atoms tothe umber of carbon atoms (F/C) is preferably from 0.05 to 0.5.Preferably a material including a fluorine-containing resin such aspolytetrafluoroethylene is present on the surface such that there is aninterface between the material and the surface of the photoreceptor.

[0060] Alternatively, a fatty acid metal salt such as zinc stearate ispresent on the surface of the photoreceptor such that there is aninterface between the material and the surface of the photoreceptor. Inthis case, it is preferable that the surface is analyzed by an XPSmethod, the ratio of the number of metal (zinc) atoms to the number ofcarbon atoms (M(Zn)/C) is from 0.001 to 0.1.

[0061] In another aspect of the present invention, an image formingapparatus including a photoreceptor, a charger which charges the surfaceof the photoreceptor, a light irradiator which irradiates thephotoreceptor with imagewise light to form an electrostatic latent imagethereon, an image developer which develops the electrostatic latentimage with a developer including a toner to form a toner image, atransfer which transfer the toner image to a receiving material and afixer which fixes the toner image on the receiving material, wherein thephotoreceptor is the photoreceptor of the present invention. The imageforming apparatus may further include a lubricant applicator whichapplies a lubricant such as materials including a fluorine atom and acarbon atom and fatty acid metal salts.

[0062] In yet another aspect of the present invention, a processcartridge which at least includes a housing and the photoreceptor of thepresent invention which is contained in the housing is provided.

[0063] These and other objects, features and advantages of the presentinvention will become apparent upon consideration of the followingdescription of the preferred embodiments of the present invention takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Various other objects, features and attendant advantages of thepresent invention will be more fully appreciated as the same becomesbetter understood from the detailed description when considered inconnection with the accompanying drawings in which like referencecharacters designate like corresponding parts throughout and wherein:

[0065]FIG. 1 is a schematic view illustrating a main part of anembodiment of the image forming apparatus of the present invention;

[0066]FIGS. 2A and 2B are schematic views illustrating an embodiment ofthe lubricant applicator for use in the image forming apparatus of thepresent invention;

[0067]FIGS. 3A and 3B are schematic views illustrating anotherembodiment of the lubricant applicator for use in the image formingapparatus of the present invention;

[0068]FIG. 4 is a schematic view illustrating yet another embodiment ofthe lubricant applicator for use in the image forming apparatus of thepresent invention;

[0069]FIG. 5 is a schematic view illustrating a further embodiment ofthe lubricant applicator for use in the image forming apparatus of thepresent invention;

[0070]FIG. 6 is a cross section of an embodiment of the photoreceptor ofthe present invention;

[0071]FIG. 7 is a cross section of another embodiment of thephotoreceptor of the present invention;

[0072]FIG. 8 is a cross section of yet another embodiment of thephotoreceptor of the present invention;

[0073]FIG. 9 is a schematic view illustrating an instrument formeasuring friction coefficient of the surface of a photoreceptor usingan Euler belt method; and

[0074]FIG. 10 is a schematic view illustrating an embodiment of theprocess cartridge of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0075] Generally, the present invention provides a photoreceptor whichincludes a photosensitive layer on which nitrate ion (NO₃ ⁻) is presentin an amount of from 50 to 300 μg per 1 m² of the surface, when measuredby an ion chromatography method. The photoreceptor having such aproperty can stably produce images having high resolution.

[0076] In addition, it is preferably that a material having a fluorineatom and a carbon atom is present on the surface of the photoreceptorand when the surface of the photoreceptor is analyzed by an XPS method,the ratio of the number of fluorine atom to the number of carbon atom(F/C) is from 0.05 to 0.5. Alternatively, a fatty acid metal salt suchas zinc stearate may be present on the surface and when the surface isanalyzed by an XPS method, the ratio of the number of metal (zinc) atomto the number of carbon atom (M(Zn)/C) is from 0.001 to 0.1.

[0077] The photoreceptor having a combination of the former property andat least one of the latter properties can stably produce images havinghigh resolution and has good durability.

[0078] The present invention also provides an image forming apparatususing the photoreceptor of the present invention. The thus preparedphotoreceptor or image forming apparatus has good durability and canstably produce images having high resolution.

[0079] When the photosensitive layer of a photoreceptor is abraded, theelectric properties of the photoreceptor, such as surface potential andphoto-decay properties, change. Therefore, images (i.e., the finaloutput) having good image qualities cannot be produced by thepredetermined processes.

[0080] The photoreceptor is abraded by contacting other units such as acleaning unit, a developing unit and a transferring unit. Among theseunits, the cleaning unit, which mechanically removes the residual tonerparticles on the photoreceptor using a blade or a brush, has a greatinfluence on the abrasion of the photoreceptor.

[0081] The abrasion of a photoreceptor in the cleaning unit isclassified into the following two types of abrasion:

[0082] (1) abrasion due to the shear strength applied to thephotoreceptor by a blade or a brush (first type abrasion); and

[0083] (2) abrasion due to toner particles which are present between thephotoreceptor and a blade or a brush and which serves like a whetstoneor sandpaper (second type abrasion).

[0084] As the factors having an influence on the abrasion are asfollows:

[0085] (1) mechanical strength of the photoreceptor;

[0086] (2) contact pressure of the cleaning blade or brush;

[0087] (3) hardness of the toner particles; and

[0088] (4) coefficient of friction (p) of the surface of thephotoreceptor.

[0089] The present inventors discover that there is a correlationbetween the first type abrasion of a photoreceptor and the shearstrength applied to the photoreceptor by a blade (or a brush). Thereforeit is discovered that by controlling the coefficient of friction of thesurface of a photoreceptor so as to be low, the abrasion of thephotoreceptor can be reduced, namely, high durability can be imparted tothe photoreceptor and image forming apparatus.

[0090] In order to decrease the coefficient of friction of the surfaceof a photoreceptor, for example, the following methods can be used:

[0091] (1) a material which decreases coefficient of friction is addedor dispersed in the surface layer of the photoreceptor; and

[0092] (2) a lubricant is applied to the surface of the photoreceptorfrom the outside.

[0093] The former method has an advantage over the latter method in thata lubricant applying device is not needed in the image formingapparatus. However, the former method has drawbacks such that thelubricating effect cannot be maintained for a long time, and when thematerial is added too much, the material adversely affects thecharacteristics of the photoreceptor.

[0094] On the contrary, the latter method has advantages such that thelubricating effect can be maintained for a long time, and the lubricanthardly affect adversely the characteristics of the photoreceptor becausethe lubricant is present only on the surface of the photoreceptor.

[0095] Next, the method for preventing a photoreceptor from beingdeteriorated by corona-discharge-induced ionic products in the chargingprocess and image transfer process will be explained in detail.

[0096] When such corona-discharge-induced ionic products adhere on thesurface of a photoreceptor, the surface resistance and bulk resistanceof the photoreceptor decrease, resulting in deterioration of thephotosensitive layer. The reason is considered to be that the ionicproducts adhere to or react with the materials in the photosensitivelayer. In particular, under high humidity conditions, water is adsorbedon the surface of the photoreceptor, and the resistance of thephotoreceptor in the surface direction decreases because the ionicproducts are present on the surface thereof. Therefore, the surfacepotential of an electrostatic latent image formed on the surface of thephotoreceptor decreases, and thereby images having good image qualitiescannot be formed. Therefore, it is necessary to control the amount ofthe corona-discharge-induced ionic products present on the surface ofthe photoreceptor so as to fall in a certain range.

[0097] The corona-discharge-induced ionic products include various ionicmaterials such as ammonium nitrate. Among these ionic materials, anitrate ion (NO₃ ⁻) is generated in a greater amount than the othermaterials. Therefore, the amount of the ionic products present on thesurface of a photoreceptor can be monitored by measuring the amount of anitrate ion.

[0098] In order to control the amount of the corona-discharge-inducedionic products present on the surface of a photoreceptor so as to fallin the range mentioned above, the following methods can be used:

[0099] (1) the voltage applied to the charging element is controlled soas to be as small as possible;

[0100] (2) the voltage is timely applied to the photoreceptor tominimize the time for charging the photoreceptor;

[0101] (3) the ionic products adhered on the photoreceptor are removedby a cleaning blade which has an appropriate hardness and to which anappropriate pressure is applied;

[0102] (4) the ionic products adhered on the photoreceptor are removedby a cleaning brush having fibers, which are made of polyester, nylon orthe like optionally subjected to an electroconductive treatment andwhich have an appropriate hardness, diameter, and density, wherein thepressure, rotation speed and rotation direction of the brush areoptimized;

[0103] (5) ionic products are removed from the photoreceptor by beingrubbed by a rotating cleaning unit without performing image formingprocesses such as a charging process and a developing process (i.e.,only an ion product removing process is performed without performingimage forming processes); and the like method.

[0104] It is important to control the amount of the nitrate ion presenton the surface of a photoreceptor, rather than which method is used.

[0105] The image forming apparatus of the present invention will beexplained in detail referring to drawings.

[0106]FIG. 1 is a schematic view illustrating a main part of the imageforming apparatus of the present invention.

[0107] In FIG. 1, numeral 1 denotes a photoreceptor having a drum shape,which rotates in a direction as indicated by an arrow. Around thephotoreceptor 1, a contact charger (or a short range charger) 2 whichcharges the photoreceptor 1; a light irradiator 3 which irradiates thecharged photoreceptor 1 with imagewise light to form an electrostaticlatent image on the photoreceptor 1; an image developer 4 which developsthe electrostatic latent image with a developer including a toner toform a toner image on the photoreceptor 1; a contact transfer 6 whichtransfers the toner image to a receiving material 5; a cleaner 7 whichremoves the residual toner particles on the surface of the photoreceptor1; a discharging lamp 8 which discharges the residual potential on thephotoreceptor 1; and a fixer 9 which fixes the toner image on thereceiving material 5, are provided.

[0108]FIG. 10 is a schematic view illustrating an embodiment of theprocess cartridge of the present invention. The process cartridge isused for image forming apparatus while being detachably attached to theapparatus.

[0109] In FIG. 10, the process cartridge includes a housing 215, aphotoreceptor 216, a charger 217, a cleaning brush 218, and a developingroller 219. The photoreceptor 216 is the photoreceptor of the presentinvention. The constitution of the process cartridge of the presentinvention is not limited thereto. The process cartridge of the presentinvention includes at least the housing 215 and the photoreceptor of thepresent invention.

[0110] FIGS. 2 to 5 are schematic views illustrating embodiments of thelubricant applicator for use in the image forming apparatus of thepresent invention.

[0111] In FIG. 2A, a lubricant is applied to the photoreceptor in thecharging process. Numerals 101 and 102 denote a photoreceptor and acontact charging roller, respectively. A part of the contact chargingroller 102 is enlarged in FIG. 2B. In FIG. 2B, numerals 111 and 112denote a charging material for charging the photoreceptor 101, and alubrication applying material for applying lubrication to the surface ofthe photoreceptor 101, respectively. The contact charging roller 102applies the lubrication applying material on the surface of thephotoreceptor 101.

[0112] In FIG. 3A, a lubricant is applied to the photoreceptor in thetransfer process. Numeral 106 denotes a transfer belt. A part of thetransfer belt 106 is enlarged in FIG. 3B. In FIG. 3B, numerals 119 and120 denote a transfer voltage applying material and a lubricationapplying material for applying lubrication to the surface of thephotoreceptor 101, respectively. The transfer belt 106 applies thelubrication applying material to the surface of the photoreceptor 101.

[0113] In FIG. 4, a lubricant applying device is provided before thecleaning unit. Numerals 107 and 113 denote a cleaning blade, and acleaning brush, respectively. A lubricant applying roller 114 applies alubricant 115 to the cleaning brush 113. The lubricant 115 is thereforeapplied to the surface of the photoreceptor 101. The lubricant 115 ispressed by a spring 116.

[0114] In FIG. 5, a lubricant applying device is provided after thecleaning unit. A lubricant applying element 117 applies a lubricant tothe surface of the photoreceptor 101 while being pressed by a spring118.

[0115] In addition, a lubricant can also be applied to the photoreceptorby using a toner including the lubricant such as fatty acid metal salts(e.g., zinc stearate) or a developer including the lubricant for theimage forming apparatus as shown in FIG. 1. In this case, it ispreferable that a replenishing toner including the lubricant or areplenishing developer including the developer, which is contained in acontainer (not shown) is supplied to the developing unit little bylittle.

[0116] The method for applying a lubricant to the surface of thephotoreceptor is not limited to the above-mentioned methods, and anymethod in which a lubricant is applied to the surface of thephotoreceptor from the outside can be employed.

[0117] Next, the image forming processes will be explained in detail.

[0118] At first, the charging process will be explained. As mentionedabove, the charging methods are classified into two types, one of whichis non-contact charging methods and the other of which is contactcharging methods.

[0119] Among the non-contact charging methods, a corona dischargingmethod is well known which charges a photoreceptor using anelectroconductive element such as wires and plates, which is providedapart from the surface of the photoreceptor and to which a high voltageis applied. This method has an advantage in that the surface of aphotoreceptor can be uniformly charged, and therefore the method hasbeen typically used.

[0120] On the contrary, in the contact charging methods, a photoreceptoris charged by a charging element, such as brushes, roller-shapedbrushes, rollers, blades and belts, which has an appropriateelectroconductivity and elasticity and which contacts the surface of thephotoreceptor. These contact charging methods have been disclosed inJapanese Laid-Open patent Publications Nos. 63-149668 and 7-281503.

[0121] The contact charging methods have an advantage over thenon-contact charging methods in that the voltage applied to thephotoreceptor can be reduced and thereby the amount of generated ozone,which is considered to damage human beings and photoreceptors, can bereduced. Therefore, these contact charging methods have widely spread.

[0122] As intermediate methods between the contact charging methods andnon-contact charging methods, short range charging methods in which a DCvoltage overlapped with a DC or AC voltage is applied to a photoreceptorusing a charging element, such as a brush, a roller-shaped brush, aroller, a blade or a belt, which has an appropriate electroconductivityand elasticity, while a narrow gap is formed between the chargingelement and the photoreceptor. The short range charging methods arepractically used recently.

[0123] The charging process is followed by a light irradiating process.The light irradiating device irradiates the charged photoreceptor withimagewise light. The imagewise light may be an analogue light imagewhich is the light image reflected from an original document and passingthrough a lens or a mirror, or a digital light image which is emitted bya laser diode and a light emitting device and which is obtained byreproducing electric signals output from a computer or signals which areobtained by reading a document by a sensor such as charge coupleddevices (CCDs). Recently, the light irradiating device irradiating adigital light image is typically used because various image processingis possible and images having good image qualities can be stablyproduced.

[0124] An electrostatic latent image formed on the photoreceptor is thendeveloped with a developing device, which contains a developer includinga toner, to form a toner image on the photoreceptor. As the developer,one component dry developers, two component dry developers and liquiddevelopers can be used.

[0125] The toner image formed on the photoreceptor is directlytransferred onto a receiving material such as paper, plastic films andthe like. The toner image on the photoreceptor is optionally transferredonto an intermediate transfer material, and then transferred onto areceiving material. In order to transfer the toner image, one or more ofthe above-mentioned non-contact charging methods using coronadischarging and contact charging methods using a roller, a brush or abelt are typically used.

[0126] After transferring the toner image, the residual toner on thephotoreceptor is removed by a cleaning unit. The cleaning unit typicallyincludes a roller-shaped brush or an elastic blade by which the residualtoner is squeezed. In recent years, there are image forming apparatuswhich do not have a cleaning unit because toner images are transferredon a receiving material with high efficiency.

[0127] The lubricant applicator, which applies a lubricant to thesurface of the photoreceptor, is classified into devices as shown inFIG. 5 which directly apply a lubricant to the surface of thephotoreceptor and devices as shown in FIG. 4 which indirectly apply alubricant to the surface of the photoreceptor.

[0128] Specific examples of such a lubricant includes lubricatingliquids such as silicone oils and fluorine-containing oils;fluorine-containing resins such as polytetrafluoroethylene (PTFE),perfluoroalkylvinyl ether (PFA) and polyvinylidene fluoride (PVDF);lubricating solids (e.g., powder) such as silicone resins, polyolefinresins, silicone grease, fluorine-containing grease, paraffin waxes,fatty acid esters, fatty acid metal salts such as zinc stearate,graphite and molybdenum disulfide; and the like.

[0129] Among these materials, fluorine-containing resins and fatty acidmetal salts are preferable because of being easy to handle and havinggood lubricating properties. Among the fluorine-containing resins, PTFEis preferable because of being easily processed into any desired shapeand decreasing the friction coefficient of the surface of thephotoreceptor.

[0130] Among the fatty acid metal salts, metal salts of palmitic acid,stearic acid and oleic acid are preferable. As the metal of the fattyacid metal salts, zinc, calcium and aluminum are preferable. Inparticular, zinc stearate and zinc palmitate are preferable.

[0131] Next, it will be explained why the content of nitrate iondetected from the surface of the photoreceptor and/or thefluorine/carbon ratio or the zinc/carbon ratio at the surface of thephotoreceptor should be controlled.

[0132] As mentioned above, when ionic products generated in variouscharging operations adhere on the surface of a photoreceptor, thesurface tends to adsorb water, resulting in decrease of the surfaceresistance of the photoreceptor.

[0133] On the other hand, in recent years, image forming apparatushaving a digital light image irradiating device using a laser diode oran LED array are widely used. In these image forming apparatus, thediameter of the light beam used for the digital light image irradiatingdevice becomes smaller and smaller to produce images having highresolution. The diameter of the light beam is about 50 μm or less nowbecause the optics used therefor are improved.

[0134] A fine electrostatic latent image which is formed using such alight beam having small diameter is sensitive to the change of thesurface resistance of the photoreceptor. Therefore, good electrostaticlatent images cannot be stably formed on the surface of such aphotoreceptor whose surface resistance easily changes depending on theenvironmental conditions such as humidity, even though goodelectrostatic latent images can be formed thereon by the conventionallight image irradiating device such as analogue light image irradiatingdevices.

[0135] In order to form good electrostatic latent images on aphotoreceptor using a digital light image irradiating device, the amountof nitrate ion present on the surface of the photoreceptor is preferablyfrom about 50 to about 300 μg per 1 m² of the surface of thephotoreceptor. The method for measuring the amount of nitrate ionpresent on the surface of a photoreceptor is explained later.

[0136] When the nitrate ion concentration on the surface of aphotoreceptor is too high, good electrostatic latent images cannot beformed on the photoreceptor especially under high humidity conditions.On the contrary, when the nitrate ion concentration is too low, thesurface potential on the photoreceptor has significant dependence onenvironmental conditions when using a contact charging method.

[0137] As mentioned above, when the friction coefficient of the surfaceof a photoreceptor is decreased, the abrasion of the surface of thephotoreceptor can be reduced. In addition, it is preferable that alubricant is applied to the surface of a photoreceptor from the outsidebecause of hardly producing adverse effect and maintaining the effectfor the long time. In this case, the friction coefficient of the surfaceof the photoreceptor depends on the amount of the lubricant present onthe surface thereof. In addition, it is important that the lubricant isnot a constituent of the photoreceptor, i.e., a clear interface ispresent between the surface of the photoreceptor and the lubricant layerformed on the surface.

[0138] As mentioned above, various lubricants can be used in the presentinvention. However, among the lubricants, fluorine-containing materialsand fatty acid metal salts are preferable because of being easy tohandle and having good lubrication imparting property, and chemicalstability.

[0139] When the friction coefficient of the surface of a photoreceptoris too large, the surface of the photoreceptor is easily abraded,resulting in shortage of the life of the photoreceptor. On the contrary,when the friction coefficient is too low, the adhesion of tonerparticles to the photoreceptor decreases, and thereby the problem whichoccurs is that a desired toner image cannot be formed on thephotoreceptor. This problem is particularly occurs when an electrostaticlatent image on a photoreceptor is developed with a two componentdeveloper while the developer contacts the surface of the photoreceptor.This is because the toner image once formed on the photoreceptor isscraped or moved by the ears of the developer.

[0140] This problem is fatal to the image forming apparatus becauseimages having high resolution cannot be produced. In order to avoid thisproblem, the friction coefficient of the photoreceptor is controlled. Asa result of the present inventors' investigation using afluorine-containing material, preferably a material having afluorine-carbon bond, as a lubricant which is applied to the surface ofa photoreceptor from the outside, it is discovered that the ratio offluorine/carbon (F/C) on the surface of the photoreceptor, when thesurface is analyzed by an XPS method, is preferably from 0.05 to 0.5 byatom. In this case, the carbon atoms present in the surface layer of thephotoreceptor is detected by the XPS method, when the lubricant layer isrelatively thin. However, it is discovered that there is a relationshipbetween the F/C ratio and resolution of the recorded images. Namely,when the F/C ratio is too large, blurred images tend to be produced.When the ratio is too small, the abrasion problem tend to occur.

[0141] In addition, as a result of the present inventors' investigationusing various fatty acid zinc salts as the lubricant, it is discoveredthat the ratio of zinc/carbon (Zn/C) on the surface of thephotoreceptor, when the surface is analyzed by an XPS method, ispreferably from 0.001 to 0.1 by atom. In this case, when the ratio isout of the range, the problems mentioned above tend to also occur.

[0142] As can be understood from the above-description, the nitrate ionconcentration is preferably controlled so as to fall in the range offrom 50 to 300 μg/m² while controlling the F/C ratio so as to fall inthe range of from 0.05 to 0.5 by atom or controlling the Zn/C ratio soas to fall in the range of from 0.01 to 0.1 by atom, to prolong the lifeof the photoreceptor and to form toner images having good resolutioneven when a light beam having small diameter is used. This is achievedby applying a lubricant such as fluorine-containing materials or fattyacid zinc salts on the surface of the photoreceptor while the nitrateion concentration is controlled by the methods mentioned above.

[0143] Then the photoreceptor for use in the present invention will beexplained in detail.

[0144] As the photosensitive layer for use in the photoreceptor of thepresent invention, for example, the following known photosensitivelayers can be used:

[0145] (1) a photosensitive layer, which is mainly constituted ofselenium or a selenium alloy;

[0146] (2) a photosensitive layer, which is mainly constituted of abinder resin and an inorganic photoconductor such as zinc oxide andcadmium sulfide;

[0147] (3) a photosensitive layer, which is mainly constituted ofamorphous silicon; and

[0148] (4) a photosensitive layer, which is mainly constituted of one ormore organic photosensitive materials.

[0149] FIGS. 6 to 8 are cross sections of embodiments of the organicphotoreceptor for use in the present invention. In FIG. 6, an undercoatlayer 25, a charge generation layer 31 and a charge transport layer 33are formed on an electroconductive substrate 21 in this order. In FIG.7, an undercoat layer 25 and a photosensitive layer 23 are formed on anelectroconductive substrate 21 in this order. In FIG. 8, an undercoatlayer 25, a charge generation layer 31, a charge transport layer 33 anda protective layer 34 are formed on an electroconductive substrate 21 inthis order. The structure of the organic photoreceptor for use in thepresent invention is not limited thereto. In the present invention, asshown in FIGS. 6 and 8, the undercoat layer 25 and the protective layer34 is considered as a layer of the photosensitive layer 23.

[0150] Suitable materials for use as the electroconductive substrate 21include materials having a volume resistance not greater than 10¹⁰ Ωcm.Specific examples of such materials include plastic cylinders, plasticfilms or paper sheets, on the surface of which a metal such as aluminum,nickel, chromium, nichrome, copper, silver, gold, platinum, iron and thelike, or a metal oxide such as tin oxides, indium oxides and the like,is deposited or sputtered. In addition, a tube can also be used as thesubstrate 21 which is prepared by tubing a plate of a metal such asaluminum, aluminum alloys, nickel, stainless steel and the like, ortubing by a method such as impact ironing or direct ironing, and thensubjecting the surface of the tube by a cutting, super finishing,polishing and/or the like treatment. Further, endless belts of a metalsuch as nickel, stainless steel and the like can also be used as thesubstrate 21.

[0151] The photosensitive layer of the photoreceptor for use in thepresent invention may be a single layer type or a multi-layer type. Atfirst the multi-layer type organic photosensitive layer will beexplained referring to the photoreceptor as shown in FIG. 6 for onlyexplanation convenience.

[0152] The charge generation layer 31 is mainly constituted of a chargegeneration material, and optionally a binder resin is used. As thecharge generation material, inorganic or organic charge generationmaterials can be used.

[0153] Specific examples of the inorganic charge generation materialsinclude crystalline selenium, amorphous selenium, selenium-telluriumalloys, selenium-tellurium-halogen alloys, selenium-arsenic alloys andamorphous silicon. Suitable amorphous silicon includes ones in which adangling bond is terminated with a hydrogen atom or a halogen atom, orin which a boron atom or a phosphorus atom is doped.

[0154] Specific examples of the organic charge generation materialsinclude phthalocyanine pigments such as metal phthalocyanine andmetal-free phthalocyanine, azulenium pigments, squaric acid methinepigments, azo pigments having a carbazole skeleton, azo pigments havinga triphenylamine skeleton, azo pigments having a diphenylamine skeleton,azo pigments having a dibenzothiophene skeleton, azo pigments having afluorenone skeleton, azo pigments having an oxadiazole skeleton, azopigments having a bisstilbene skeleton, azo pigments having adistyryloxadiazole skeleton, azo pigments having a distyrylcarbazoleskeleton, perylene pigments, anthraquinone pigments, polycyclic quinonepigments, quinoneimine pigments, diphenyl methane pigments, triphenylmethane pigments, benzoquinone pigments, naphthoquinone pigments,cyanine pigments, azomethine pigments, indigoid pigments,bisbenzimidazole and the like materials.

[0155] These charge transport materials can be used alone or incombination.

[0156] Specific examples of the binder resin, which is optionally usedin the charge generation layer 31, include polyamide resins, polyurethane resins, epoxy resins, polyketone resins, polycarbonate resins,silicone resins, acrylic resins, polyvinyl butyral resins, polyvinylformal resins, polyvinyl ketone resins, polystyrene resins,poly-N-vinylcarbazole resins, polyacrylamide resins, and the like. Thesebinder resins can be used alone or in combination. In addition, one ormore charge transport materials maybe included in the charge generationlayer 31.

[0157] Charge transport materials may be added in the charge generationlayer 31. Specific examples of such charge transport materials includepositive hole transport materials and electron transport materials.

[0158] Specific examples of such electron transport materials includeelectron accepting materials such as chloranil, bromanil,tetracyanoethylene, tetracyanoquinodimethane,2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone,2,4,5,7-tetranitro-xanthone, 2,4,8-trinitrothioxanthone,2,6,8-trinitro-4H-indeno[1,2-b]thiophene-4-one,1,3,7-trinitrobenzothiophene-5,5-dioxide, and the like compounds. Theseelectron transport materials can be used alone or in combination.

[0159] Specific examples of such positive hole transport materialsinclude electron donating materials such as oxazole derivatives,oxadiazole derivatives, imidazole derivatives, triphenylaminederivatives, 9-(p-diethylaminostyrylanthracene),1,1-bis(4-dibenzylaminophenyl)propane, styrylanthracene,styrylpyrazoline, phenylhydrazone compounds, α-phenylstilbenederivatives, thiazole derivatives, triazole derivatives, phenazinederivatives, acridine derivatives, benzofuran derivatives, benzimidazolederivatives, thiophene derivatives, and the like. These positive holetransport materials can be used alone or in combination.

[0160] Suitable methods for forming the charge generation layer 31include thin film forming methods in a vacuum, and casting methods.

[0161] Specific examples of such thin film forming methods in a vacuuminclude vacuum evaporation methods, glow discharge decompositionmethods, ion plating methods, sputtering methods, reaction sputteringmethods, CVD (chemical vapor deposition) methods, and the like methods.A layer of the above-mentioned inorganic and organic materials can beformed by one of these methods.

[0162] The casting methods useful for forming the charge generationlayer 35 include, for example, the following steps:

[0163] (1) preparing a coating liquid by mixing one or more inorganic ororganic charge generation materials mentioned above with a solvent suchas tetrahydrofuran, cyclohexanone, dioxane, dichloroethane, butanone andthe like, and if necessary, together with a binder resin and anadditives, and then dispersing the materials with a ball mill, anattritor, a sand mill or the like;

[0164] (2) coating on a substrate the coating liquid, which is dilutedif necessary, by a dip coating method, a spray coating method, a beadcoating method, a ring coating method or the like method; and

[0165] (3) drying the coated liquid to form a charge generation layer.

[0166] The thickness of the charge generation layer 31 is preferablyfrom about 0.01 to about 5 μm, and more preferably from about 0.05 toabout 2 μm.

[0167] Next, the charge transport layer 33 will be explained in detail.

[0168] The charge transport layer 33 transports the carriers which areselectively generated in the charge generation layer 31 by irradiatingthe photosensitive layer with imagewise light to form an electrostaticlatent image on the surface of the photoreceptor. The charge transportlayer may be a layer which includes one or more of the low molecularweight charge transport materials mentioned above for use in the chargegeneration layer 31 together with a binder resin; or a layer mainlyincluding one or more high molecular weight charge transport materials(i.e., charge transport polymer materials). The charge transport layer33 is typically prepared by coating a coating liquid in which theabove-mentioned materials are dissolved or dispersed in a solvent, andthen drying the coated liquid.

[0169] Specific examples of the binder resins which are used incombination with the low molecular weight charge transport materialsinclude polycarbonate resins such as bisphenol A type and bisphenol Ztype polycarbonate resins, polyester resins, methacrylic resins, acrylicresins, polyethylene resins, vinyl chloride resins, vinyl acetateresins, polystyrene resins, phenolic resins, epoxy resins, polyurethaneresins, polyvinylidene chloride resins, alkyd resins, silicone resins,polyvinyl carbazole resins, polyvinyl butyral resins, polyvinyl formalresins, polyacrylate resins, polyacrylamide resins, phenoxy resins, andthe like resins. These binder resins can be used alone or incombination.

[0170] As the high molecular weight charge transport material, thefollowing known charge transport polymer materials (i.e., polymershaving an electron donating group) can be used:

[0171] (a) polymers having a carbazole ring in their main chain and/orside chain Specific examples of such materials include poly-N-vinylcarbazole, and compounds disclosed in Japanese Laid-Open PatentPublications Nos. 50-82056, 54-9632, 54-11737, and 4-183719.

[0172] (b) polymers having a hydrazone skeleton in their main chainand/or side chain

[0173] Specific examples of such materials include compounds disclosedin Japanese Laid-Open Patent Publications Nos. 57-78402 and 3-50555.

[0174] (c) Polysilylene Compounds

[0175] Specific examples of such materials include polysilylenecompounds disclosed in Japanese Laid-Open Patent Publications Nos.63-285552, 5-19497 and 5-70595.

[0176] (d) Polymers having a tertiary amine skeleton in their main chainand/or side chain

[0177] Specific examples of such materials includeN,N-bis(4-methylphenyl)-4-aminopolystyrene, and compounds disclosed inJapanese Laid-Open Patent Publications Nos. 1-13061, 1-19049, 1-1728,1-105260, 2-167335, 5-66598 and 5-40350.

[0178] (e) Other Polymers

[0179] Specific examples of such materials include condensation productsof nitropyrene with formaldehyde, and compounds disclosed in JapaneseLaid-Open Patent Publications Nos. 51-73888 and 56-150749.

[0180] The high molecular weight charge transport polymer material(polymer having an electron donating group) for use in the chargetransport layer 33 is not limited thereto, and known copolymers (random,block and graft copolymers) and star polymers, which have an electrondonating group, and crosslinking polymers having an electron donatinggroup disclosed in, for example, Japanese Laid-Open Patent PublicationNo. 3-109406 can also be used.

[0181] The high molecular weight charge transport material is optionallyused together with a binder resin, a low molecular weight chargetransport material and/or additives such as plasticizers and levelingagents.

[0182] Specific examples of the plasticizers include known plasticizers,which have been used for plasticizing a resin, such as dibutylphthalate, and dioctyl phthalate. The content of the plasticizer in thecharge transport layer is preferably from 0 to 30 parts by weight per100 parts by weight of the binder resin (and/or charge transport polymermaterial) included in the layer.

[0183] Specific examples of the leveling agents include silicone oilssuch as dimethyl silicone oils and methylphenyl silicone oils; andpolymers and oligomers having a perfluoroalkyl group in their sidechain. The content of the leveling agent in the charge transport layeris preferably from 0 to 1 part by weight per 100 parts by weight of thebinder resin (and/or charge transport polymer material) included in thelayer.

[0184] The thickness of the charge transport layer 33 is preferably from5 to 100 μm, and more preferably from 10 to 40 μm.

[0185] Then the single layer type photosensitive layer 23 will beexplained referring to FIG. 7.

[0186] The photosensitive layer 23 is typically formed by coating acoating liquid including a charge generation material, and a lowmolecular weight charge transport material and/or a charge transportpolymer material. The above-mentioned charge generation materials, lowmolecular weight charge transport materials and charge transport polymermaterials for use in the charge generation layer 31 and charge transportlayer 33 can also be used in the photosensitive layer 23.

[0187] The photosensitive layer 23 optionally includes a binder resin,and/or additives such as plasticizers and leveling agents. Specificexamples of the binder resin, plasticizers and leveling agents includethe materials mentioned above for use in the charge generation layer 31and charge transport layer 33. The thickness of the photosensitive layer23 is preferably from 5 to 100 μm, and more preferably from 10 to 40 μm.

[0188] The photoreceptor of the present invention may include theundercoat layer 25 which is formed between the electroconductivesubstrate 21 and the photosensitive layer 23 or the charge generationlayer 31. The undercoat layer is formed, for example, to prevent moirein the resultant image, to decrease residual potential in the resultantphotoreceptor, and to prevent charge injection from the substrate to thephotosensitive layer, and to improve the coating quality of the upperlayer (i.e., to form a uniform layer of the photosensitive layer 23 orthe charge generation layer 31).

[0189] The undercoat layer 25 mainly includes a resin. Since aphotosensitive layer coating liquid, which typically includes an organicsolvent, is coated on the undercoat layer, the resin used in theundercoat layer preferably has good resistance to popular organicsolvents.

[0190] Specific examples of such resins for use in the undercoat layerinclude water-soluble resins such as polyvinyl alcohol, casein andpolyacrylic acid; alcohol-soluble resins such as nylon copolymers, andmethoxymethylated nylons; and crosslinkable resins such as polyurethaneresins, melamine resins, alkyd-melamine resins, and epoxy resins. Inaddition, the undercoat layer may include a fine powder such as metaloxides (e.g., titanium oxide, silica, alumina, zirconium oxide, tinoxide, and indium oxide), metal sulfides, and metal nitrides. When theundercoat layer 25 is formed using these materials, known coatingmethods using a proper solvent can be used similarly to thephotosensitive layer.

[0191] In addition, a metal oxide layer which is formed, for example, bya sol-gel method using a silane coupling agent, titanium coupling agentor a chromium coupling agent can also be used as the undercoat layer.

[0192] Further, a layer of aluminum oxide which is formed by an anodicoxidation method, and a layer of an organic compound such aspolyparaxylylene or an inorganic compound such as SiO, SnO₂, TiO₂, ITOor CeO₂, which is formed by a vacuum evaporation method, are alsopreferably used as the undercoat layer.

[0193] The photoreceptor of the present invention may include theprotective layer 34 on the photosensitive layer (the photosensitivelayer 23 or charge transport layer 33) to protect the photosensitivelayer and to improve the durability of the photoreceptor. Specificexamples of the materials for use in the protective layer 34 include ABSresins, ACS resins, olefin-vinyl monomer copolymers, chlorinatedpolyethers, aryl resins, phenolic resins, polyacetal resins, polyamideresins, polyamideimide resins, polyacrylate resins, polyarylsulfoneresins, polybutylene resins, polybutyleneterephthalate resins,polycarbonate resins, polyethersulfone resins, polyethylene resins,polyethyleneterephthalate resins, polyimide resins, acrylic resins,polymethylpentene resins, polypropylene resins, polyphenylene oxideresins, polysulfone resins, polystyrene resins, AS resins,butadiene-styrene copolymers, polyurethane resins, polyvinyl chlorideresins, polyvinylidene chloride resins, epoxy resins and the likeresins.

[0194] The protective layer 34 may include a filler to improve theabrasion resistance. Specific examples of such a filler includeparticulate fluorine-containing resins such as polytetrafluoroethyleneand silicone resins. In addition, an inorganic material such as titaniumoxides, tin oxides, potassium titanate and the like can be included inthe resins.

[0195] The content of the filler in the protective layer 34 ispreferably from 10 to 40% by weight and more preferably from 20 to 30%by weight. When the content is too low, abrasion resistance cannot beimproved. When the content is too high, the surface potential of thephotoreceptor becomes high after the photoreceptor is exposed toimagewise light, resulting in occurrence of problems such as backgrounddeveloping of images due to the deterioration of photosensitivity of thephotoreceptor.

[0196] In order to improve the dispersion property of the filler,dispersion promoters can be used. Suitable dispersion promoters includeknown dispersion promoters for use in the paint. The content of thefiller in the protective layer 34 is from 0.5 to 4% by weight, andpreferably from 1 to 2% by weight of the filler included in theprotective layer.

[0197] In addition, it is preferable to add one of the charge transportmaterials mentioned above to the protective layer 34. Further, theprotective layer 34 may include one of the antioxidants mentioned below.

[0198] The protective layer 34 is typically formed by a coating methodsuch as spray coating methods. The thickness of the protective layer 34is preferably from 0.5 to 10 μm and more preferably from 4 to 6 μm.

[0199] In the present invention, an intermediate layer (not shown infigures) may be formed between the photosensitive layer 23 (or thecharge transport layer 33) and the protective layer 34. The intermediatelayer mainly includes a resin such as polyamide resins, alcohol-solublenylon resins, water-soluble butyral resins, polyvinyl butyral resins,polyvinyl alcohol resins and the like resins. This intermediate layercan also be formed by any one of the known coating methods as mentionedabove. The thickness of such intermediate layer is preferably from 0.05to 2 μm.

[0200] In the photoreceptor of the present invention, one or moreantioxidants can be used in one or more of the layers including anorganic material, to improve the dependency of the photoreceptor onenvironmental conditions, i.e., to prevent deterioration ofphotosensitivity and increase of residual potential. In particular, goodresults can be obtained when an antioxidant is included in the layerincluding a charge transport material.

[0201] Suitable antioxidants for use in the photoreceptor include thefollowing compounds, but are not limited thereto.

[0202] Monophenol Compounds

[0203] 2,6-di-t-butyl-p-cresol, butylated hydroxyanisole,2,6-di-t-butyl-4-ethylphenol,stearyl-β-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, and the likecompounds;

[0204] Bisphenol Compounds

[0205] 2,2′-methylene-bis-(4-methyl-6-t-butylphenol),2,2′-methylene-bis-(4-ethyl-6-t-butylphenol),4,4′-thiobis-(3-methyl-6-t-butylphenol),4,4′-butylidenebis-(3-methyl-6-t-butylphenol), and the like compounds;

[0206] High Molecular Phenolic Compounds

[0207] 1,1,3-tris-(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,tetrakis-[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane,bis[3,3′-bis(4′-hydroxy-3′-t-butylphenyl)butyric acid]glycol ester,tocophenol compounds, and the like compounds.

[0208] Paraphenylenediamine Compounds

[0209] N-phenyl-N′-isopropyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N-phenyl-N-sec-butyl-p-phenylenediamine,N,N′-di-isopropyl-p-phenylenediamine,N,N′-dimethyl-N,N′-di-t-butyl-p-phenylenediamine, and the likecompounds.

[0210] Hydroguinone Compounds

[0211] 2,5-di-t-octylhydroquinone, 2,6-didodecylhydroquinone,2-dodecylhydroquinone, 2-dodecyl-5-chlorohydroquinone,2-t-octyl-5-methylhydroquinone, 2-(2-octadecenyl)-5-methylhydroquinone,and the like compounds.

[0212] Sulfur-Containing Organic Compounds

[0213] dilauryl-3,3′-thiodipropionate, distearyl-3,3′-thiodipropionate,ditetradecyl-3,3′-thiodipropionate, and the like compounds.

[0214] Phosphorus-Containing Organic Compounds

[0215] triphenylphosphine, tri(nonylphenyl)phosphine,tri(dinonylphenyl)phosphine, tricresylphosphine,tri(2,4-dibutylphenoxy)phosphine, and the like compounds.

[0216] These compounds are known as antioxidants for use in rubbers,plastics, and oils and fats, and are commercially available.

[0217] The content of the antioxidant in the photosensitive layer (orprotective layer) is from 0.1 to 100 parts by weight, and preferablyfrom 2 to 30 parts by weight, per 100 parts by weight of the chargetransport material included in the layer.

[0218] Having generally described this invention, further understandingcan be obtained by reference to certain specific examples which areprovided herein for the purpose of illustration only and are notintended to be limiting. In the descriptions in the following examples,the numbers represent weight ratios in parts, unless otherwisespecified.

EXAMPLES

[0219] Preparation of Photoreceptor 1

[0220] Preparation of Undercoat Layer

[0221] The following components were mixed and dispersed to prepare anundercoat layer coating liquid. Alkyd resin 6 (tradenamed as Bekkozol1307-60-EL and manufactured by Dainippon Ink and Chemicals, Inc.)Melamine resin 4 (tradenamed as Super Bekkamin G-821-60 and manufacturedby Dainippon Ink and Chemicals, Inc.) Titanium oxide 40 Methyl ethylketone 200

[0222] The undercoat layer coating liquid was coated on the surface ofan aluminum drum having a diameter of 30 mm, and dried. Thus anundercoat layer having a thickness of 3.5 μm was prepared.

[0223] Preparation of Charge Generation Layer

[0224] The following components were mixed and dispersed to prepare acharge generation layer coating liquid. Trisazo pigment having thefollowing formula 2.5

Polyvinyl butyral 0.25 (tradenamed as XYHL and manufactured by UnionCarbide Corp.) Cyclohexanone 200 Methyl ethyl ketone 80

[0225] The charge generation layer coating liquid was coated on theundercoat layer and then dried. Thus a charge generation layer having athickness of 0.2 μm was prepared.

[0226] Preparation of Charge Transport Layer

[0227] The following components were mixed and dispersed to prepare acharge transport layer coating liquid. Bisphenol A type polycarbonateresin 10 (tradenamed as Panlite K1300 and manufactured by Teijin Ltd.)Low molecular weight charge transport material 10 having the followingformula

Methylene chloride 100

[0228] The charge transport layer coating liquid was coated on thecharge generation layer and then dried. Thus a charge transport layerhaving a thickness of 25 μm was formed.

[0229] Thus a photoreceptor 1 was prepared.

[0230] Preparation of Photoreceptor 2

[0231] The procedure for preparation of the photoreceptor 1 was repeatedexcept that the formulation of the charge generation layer coatingliquid was changed to the following.

[0232] Charge Generation Layer Coating Liquid

[0233] The following components were-mixed and dispersed using a ballmill. Y-form oxotitanylphthalocyanine pigment 2 Polyvinyl butyral resin0.2 (tradenamed as S-lec BM-S and manufactured by Sekisui Chemical Co.,Ltd.) Tetrahydrofuran 50

[0234] Thus a photoreceptor 2 was prepared.

[0235] Preparation of Photoreceptor 3

[0236] The procedure for preparation of the photoreceptor 1 wasrepeated. In addition, the following protective layer coating liquid wasprepared.

[0237] Protective Layer Coating Liquid Charge transport material having2 the following formula

A-form polycarbonate resin 4 Methylene chloride 100

[0238] The protective layer coating liquid was coated on the chargetransport layer and then dried. Thus a protective layer having athickness of 2 μm was prepared.

[0239] Thus a photoreceptor 3 was prepared.

[0240] Preparation of Photoreceptor 4

[0241] The procedure for preparation of the photoreceptor 3 was repeatedexcept that the formulation of the protective layer coating liquid waschanged to the following.

[0242] Protective Layer Coating Liquid Charge transport material having4 the following formula

A-form polycarbonate resin 4 Titanium oxide 1 Methylene chloride 100

[0243] Thus a photoreceptor 4 was prepared.

[0244] Preparation of Photoreceptor 5

[0245] The procedure for preparation of the photoreceptor 4 was repeatedexcept that the titanium oxide in the protective layer coating liquidwas replaced with aluminum oxide.

[0246] Thus a photoreceptor 5 was prepared.

[0247] These photoreceptors 1 to 5 were evaluated as follows:

[0248] (1) Running Test

[0249] Each of the photoreceptors 1 to 5 was set in a digital copier asshown in FIG. 1, Imagio MF200 manufactured by Ricoh Co., Ltd., in whicha lubricant applying device can be provided and the charging method canbe changed, and a running test in which 200,000 copies were produced atthe most. When a running test is started, the potential VD (i.e., thepotential of the photoreceptor which was not exposed to imagewise light)was set so as to be 850 V, and the potential VL (i.e., the potential ofthe lighted photoreceptor) was set so as to be, 120 V.

[0250] In the running test, the image qualities of the copies, and thefriction coefficient and abrasion of the surface of the photosensitivelayer were evaluated from time to time.

[0251] 1) Image Qualities

[0252] The image quality of a copy image was evaluated while consideringthe image density, reproducibility of fine line images, and whetherthere were undesired images.

[0253] The image quality was graded as follows:

[0254] ⊚: Excellent

[0255] ∘: Good

[0256] Δ1: Image density is slightly low

[0257] Δ2: A few small black streaks and slight background developmentare observed in the image

[0258] Δ3: Slight tailing is observed in the image

[0259] X1: Image density is significantly low

[0260] X2: Black streaks and background development are observed in theimage

[0261] X3: Tailing is observed in the image

[0262] 2) Friction Coefficient

[0263] The coefficient of static friction of the surface of the toplayer (charge transport layer or protective layer) was measured by amethod using an Euler belt.

[0264] The measuring instrument for use in the Euler belt method isshown in FIG. 9.

[0265] A character S′ denotes a paper to be measured which have a middlethickness. Two hooks are set at each end of the paper S′, and a load w(100 g) is set at one hook and a digital force gauge DS is set at theother hook. The paper S′ is set in the measuring instrument so as tocontact a photoreceptor 1A, as shown in FIG. 9. The paper S′ is pulledwith the digital force gauge DS. Provided when a force at which thepaper S′ starts to move is F, the coefficient of static friction of thephotoreceptor 1A is determined by the following equation:

μs=(π/2)ln(F/w)

[0266] wherein μs is the coefficient of static friction of thephotoreceptor 1A, F is the measured value of the force, and w is theload (gram-force).

[0267] 3) Amount of Abrasion

[0268] The abrasion amount Δd of a photosensitive layer was determinedby the following equation:

Δd=di−dl

[0269] Wherein di represents the total thickness of the photosensitivelayer before the running test and dl represents the total thickness ofthe photosensitive layer after the running test.

[0270] (2) Amount of nitrate ion on the surface of photoreceptor

[0271] The concentration of nitrate ion adhered on the surface of aphotoreceptor was measured by the following method:

[0272] (a) the surface of a photoreceptor is wiped with a non-wovenfabric wetted with distilled water;

[0273] (b) then the non-woven fabric is dipped into distilled water andsubjected to an ultrasonic vibration treatment to extract the materialsadhered to the non-woven fabric therefrom;

[0274] (c) distilled water is added to the distilled water including theextracted materials such that the solution has a predetermined volume;

[0275] (d) the amount of nitrate ion in the solution is determined usingan ion chromatograph apparatus (tradenamed as IC-7000P and manufacturedby Yokogawa Electric Corp.); and

[0276] (e) the amount of nitrate ion per a unit area (1 m²) of thesurface of the photoreceptor is determined.

[0277] (3) Fluorine/Carbon (F/C) Ratio

[0278] The F/C ratio of the surface of the photoreceptor, which relatesto the amount of the lubricant (fluorine-containing material) present onthe surface of the photoreceptor was determined by X-ray photoelectronspectroscopy (XPS). The measuring conditions were as follows:

[0279] Measuring instruments: Scanning X-ray photoelectron spectroscopicapparatus, Quantum 2000 manufactured by PHI

[0280] X-ray source: Al Kα

[0281] Scanning area: 100 μm×100 μm

[0282] (4) Zinc/Carbon (Zn/C) Ratio

[0283] The Zn/C ratio of the surface of the photoreceptor which relatesto the amount of the lubricant (fatty acid zinc salt) present on thesurface of the photoreceptor was determined by X-ray photoelectronspectroscopy (XPS). The measuring conditions were as follows:

[0284] Measuring instruments: Scanning X-ray photoelectron spectroscopicapparatus, Quantum 2000 manufactured by PHI

[0285] X-ray source: Al Kα

[0286] Scanning area: 100 μm×100 μm

EXAMPLE 1

[0287] The photoreceptor 1 was set in the image forming apparatus(modified Imagio MF200) to perform the running test mentioned above. Theimage forming conditions were as follows:

[0288] Charging method: contact charging method using a roller andapplying DC voltage

[0289] Cleaning element: cleaning blade (as shown in FIG. 1)

[0290] Lubricant applying device: not used

[0291] The results are shown in Table 1.

EXAMPLE 2

[0292] The procedure for the running test performed in Example 1 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 2.

[0293] The results are also shown in Table 1.

EXAMPLE 3

[0294] The procedure for the running test performed in Example 1 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0295] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 1

[0296] The procedure for the running test performed in Example 1 wasrepeated except that the cleaning blade was replaced with a cleaningbrush using an electroconductive nylon fiber.

[0297] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 2

[0298] The procedure for the running test performed in ComparativeExample 1 was repeated except that the photoreceptor 1 was replaced withthe photoreceptor 2.

[0299] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 3

[0300] The procedure for the running test performed in ComparativeExample 1 was repeated except that the photoreceptor 1 was replaced withthe photoreceptor 3.

[0301] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 4

[0302] The procedure for the running test performed in Example 1 wasrepeated except that a cleaning brush using a polyester fiber wasadditionally provided as the cleaning element in the image formingapparatus as shown in FIG. 1.

[0303] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 5

[0304] The procedure for the running test performed in ComparativeExample 4 was repeated except that the photoreceptor 1 was replaced withthe photoreceptor 2.

EXAMPLE 4

[0305] The procedure for the running test performed in Example 1 wasrepeated except that the lubricant applying device as shown in FIG. 5was provided in the image forming apparatus as shown in FIG. 1. Theconditions of the lubricant applying device were as follows:

[0306] Lubricant: Polytetrafluoroethylene (PTFE)

[0307] Contact pressure of the lubricant 117: 30 g

[0308] The contact pressure was measured as follows:

[0309] (1) a paper sheet (Ricopy PPC paper TYPE 6200 sold by Ricoh Co.,Ltd.) having a width of 30 mm is inserted between the element 117 andthe photoreceptor 1; and

[0310] (2) the paper sheet was pulled with a force gauge to measure theforce by which the paper starts to be moved.

[0311] The results are also shown in Table 1.

EXAMPLE 5

[0312] The procedure for the running test performed in Example 4 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 2.

[0313] The results are also shown in Table 1.

EXAMPLE 6

[0314] The procedure for the running test performed in Example 4 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0315] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 6

[0316] The procedure for the running test performed in Example 4 wasrepeated except that the contact pressure was changed to 5 g.

[0317] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 7

[0318] The procedure for the running test performed in ComparativeExample 6 was repeated except that the photoreceptor 1 was replaced withthe photoreceptor 2.

[0319] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 8

[0320] The procedure for the running test performed in Example 4 wasrepeated except that the contact pressure was changed to 150 g.

[0321] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 9

[0322] The procedure for the running test performed in ComparativeExample 8 was repeated except that the photoreceptor 1 was replaced withthe photoreceptor 2.

[0323] The results are also shown in Table 1.

EXAMPLE 7

[0324] The procedure for the running test performed in Example 1 wasrepeated except that the lubricant applying device as shown in FIG. 4was provided in the image forming apparatus as shown in FIG. 1. Theconditions of the lubricant applying device were as follows:

[0325] Lubricant: Polytetrafluoroethylene (PTFE)

[0326] Contact pressure of the lubricant 115: 10 g

[0327] (The contact pressure was measured in the same method asmentioned in Example 4)

[0328] The results are also shown in Table 1.

EXAMPLE 8

[0329] The procedure for the running test performed in Example 7 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 2.

[0330] The results are also shown in Table 1.

EXAMPLE 9

[0331] The procedure for the running test performed in Example 7 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0332] The results are also shown in Table 1.

EXAMPLE 10

[0333] The procedure for the running test performed in Example 7 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 4.

[0334] The results are also shown in Table 1.

EXAMPLE 11

[0335] The procedure for the running test performed in Example 7 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 5.

[0336] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 10

[0337] The procedure for the running test performed in Example 7 wasrepeated except that the contact pressure of the lubricant 115 waschanged to 2 g.

[0338] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 11

[0339] The procedure for the running test performed in ComparativeExample 10 was repeated except that photoreceptor 1 was changed to thephotoreceptor 2.

[0340] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 12

[0341] The procedure for the running test performed in Example 7 wasrepeated except that the contact pressure of the lubricant 115 waschanged to 50 g.

[0342] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 13

[0343] The procedure for the running test performed in ComparativeExample 12 was repeated except that photoreceptor 1 was changed to thephotoreceptor 2.

[0344] The results are also shown in Table 1.

EXAMPLE 12

[0345] The procedure for the running test performed in Example 1 wasrepeated except that the toner in the two component developer waschanged to the following:

[0346] Toner: a zinc stearate powder was added to the toner in an amountof 0.05 parts per 1 part by weight of the toner

[0347] The replenishing toner was also replaced with the toner mentionedabove.

[0348] The results are also shown in Table 1.

EXAMPLE 13

[0349] The procedure for the running test performed in Example 12 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 2.

[0350] The results are also shown in Table 1.

EXAMPLE 14

[0351] The procedure for the running test performed in Example 12 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0352] The results are also shown in Table 1.

EXAMPLE 15

[0353] The procedure for the running test performed in Example 12 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 4.

[0354] The results are also shown in Table 1.

EXAMPLE 16

[0355] The procedure for the running test performed in Example 12 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 5.

[0356] The results are also shown in Table 1.

COMPARATIVE EXAMPLE 14

[0357] The procedure for the running test performed in Example 12 wasrepeated except that the ratio of the zinc stearate to the toner waschanged to 0.3/1 by weight.

[0358] The results are also shown in Table 1.

EXAMPLE 17

[0359] The procedure for the running test performed in Example 1 wasrepeated except that the toner in the two component developer waschanged to the following:

[0360] Toner: a zinc stearate powder was added to the toner in an amountof 0.3 parts per 1 part by weight of the toner

[0361] The replenishing toner was also replaced with the toner mentionedabove.

[0362] In addition, a cleaning brush using a polyester fiber wasadditionally provided to the cleaning unit.

[0363] The results are also shown in Table 1.

EXAMPLE 18

[0364] The procedure for the running test performed in Example 17 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0365] The results are also shown in Table 1.

EXAMPLE 19

[0366] The procedure for the running test performed in Example 17 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 4.

[0367] The results are also shown in Table 1.

EXAMPLE 20

[0368] The procedure for the running test performed in Example 17 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 5.

[0369] The results are also shown in Table 1.

EXAMPLE 21

[0370] The procedure for the running test performed in Example 17 wasrepeated except that the charging device was changed to the followingshort range charging device:

[0371] (1) A tape having a thickness of 50 μm was adhered on both sidesof the photoreceptor 1 to form a gap between the photoreceptor 1 and thecharging roller; and,

[0372] (2) A DC voltage of −750 V was applied to the charging rollerwhile an AC voltage having a frequency of 1 KHz and a peak-to-peakvoltage of 1.5 KV was overlapped.

[0373] The results are also shown in Table 1.

EXAMPLE 22

[0374] The procedure for the running test performed in Example 21 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 3.

[0375] The results are also shown in Table 1.

EXAMPLE 23

[0376] The procedure for the running test performed in Example 21 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 4.

[0377] The results are also shown in Table 1.

EXAMPLE 24

[0378] The procedure for the running test performed in Example 21 wasrepeated except that the photoreceptor 1 was replaced with thephotoreceptor 5.

[0379] The results are also shown in Table 1. TABLE 1 Nitrate ion F/CZn/C Abrasion Image (μg/m²) ratio ratio (μm) qualities Photoreceptor inan initial state Ex. 1 50 0 0 0.0 ⊚ Ex. 2 50 0 0 0.0 ⊚ Ex. 3 50 0 0 0.0⊚ Comp. Ex. 1 50 0 0 0.0 ⊚ Comp. Ex. 2 50 0 0 0.0 ⊚ Comp. Ex. 3 50 0 00.0 ⊚ Comp. Ex. 4 50 0 0 0.0 ⊚ Comp. Ex. 5 50 0 0 0.0 ⊚ Ex. 4 50 0 0 0.0⊚ Ex. 5 50 0 0 0.0 ⊚ Ex. 6 50 0 0 0.0 ⊚ Comp. Ex. 6 50 0 0 0.0 ⊚ Comp.Ex. 7 50 0 0 0.0 ⊚ Comp. Ex. 8 50 0 0 0.0 ⊚ Comp. Ex. 9 50 0 0 0.0 ⊚ Ex.7 50 0 0 0.0 ⊚ Ex. 8 50 0 0 0.0 ⊚ Ex. 9 50 0 0 0.0 ⊚ Ex. 10 50 0 0 0.0 ⊚Ex. 11 50 0 0 0.0 ⊚ Comp. Ex. 10 50 0 0 0.0 ⊚ Comp. Ex. 11 50 0 0 0.0 ⊚Comp. Ex. 12 50 0 0 0.0 ⊚ Comp. Ex. 13 50 0 0 0.0 ⊚ Ex. 12 50 0 0 0.0 ⊚Ex. 13 50 0 0 0.0 ⊚ Ex. 14 50 0 0 0.0 ⊚ Ex. 15 50 0 0 0.0 ⊚ Ex. 16 50 00 0.0 ⊚ Comp. Ex. 14 50 0 0 0.0 ⊚ Ex. 17 50 0 0 0.0 ⊚ Ex. 18 50 0 0 0.0⊚ Ex. 19 50 0 0 0.0 ⊚ Ex. 20 50 0 0 0.0 ⊚ Ex. 21 50 0 0 0.0 ⊚ Ex. 22 500 0 0.0 ⊚ Ex. 23 50 0 0 0.0 ⊚ Ex. 24 50 0 0 0.0 ⊚ Photoreceptor after100,000 copies Ex. 1 80 0.00 0.000 8.0 ⊚ Ex. 2 70 0.00 0.000 7.0 ⊚ Ex. 390 0.00 0.000 5.0 ⊚ Comp. Ex. 1 350 0.00 0.000 0.2 ×3 Comp. Ex. 2 3500.00 0.000 0.2 ×3 Comp. Ex. 3 360 0.00 0.000 0.2 ×3 Comp. Ex. 4 30 0.000.000 12.0 Δ2 Comp. Ex. 5 30 0.00 0.000 13.0 Δ2 Ex. 4 150 0.25 0.000 1.0⊚ Ex. 5 180 0.26 0.000 0.9 ⊚ Ex. 6 200 0.27 0.000 0.6 ⊚ Comp. Ex. 6 1000.03 0.000 7.0 ⊚ Comp. Ex. 7 80 0.03 0.000 7.0 ⊚ Comp. Ex. 8 400 0.550.000 0.2 ×3 Comp. Ex. 9 450 0.60 0.000 0.2 ×3 Ex. 7 160 0.26 0.000 0.9⊚ Ex. 8 160 0.26 0.000 1.0 ⊚ Ex. 9 150 0.25 0.000 0.7 ⊚ Ex. 10 180 0.270.000 0.2 ⊚ Ex. 11 170 0.26 0.000 0.2 ⊚ Comp. Ex. 10 80 0.03 0.000 8.0 ⊚Comp. Ex. 11 70 0.03 0.000 7.0 ⊚ Comp. Ex. 12 500 0.60 0.000 0.2 ×3Comp. Ex. 13 550 0.62 0.000 0.2 ×3 Ex. 12 60 0.00 0.002 1.8 ⊚ Ex. 13 700.00 0.002 1.9 ⊚ Ex. 14 80 0.00 0.005 0.9 ⊚ Ex. 15 140 0.00 0.050 0.2 ⊚Ex. 16 150 0.00 0.050 0.2 ⊚ Comp. Ex. 14 580 0.00 0.150 0.1 ×3 Ex. 17160 0.00 0.040 0.9 ⊚ Ex. 18 180 0.00 0.050 0.5 ⊚ Ex. 19 200 0.00 0.0600.2 ⊚ Ex. 20 210 0.00 0.060 0.2 ⊚ Ex. 21 220 0.00 0.040 1.2 ⊚ Ex. 22 2400.00 0.050 0.6 ⊚ Ex. 23 250 0.00 0.060 0.2 ⊚ Ex. 24 250 0.00 0.060 0.2 ⊚Photoreceptor after 200,000 copies Ex. 1 80 0.00 0.000 15.0 Δ2 Ex. 2 800.00 0.000 14.0 Δ2 Ex. 3 95 0.00 0.000 12.0 Δ2 Comp. Ex. 1 450 0.000.000 0.5 ×3 Comp. Ex. 2 470 0.00 0.000 0.4 ×3 Comp. Ex. 3 480 0.000.000 0.3 ×3 Comp. Ex. 4 30 0.00 0.000 23.0 ×3 Comp. Ex. 5 30 0.00 0.00024.0 ×3 Ex. 4 200 0.28 0.000 2.0 ⊚ Ex. 5 220 0.29 0.000 2.0 ⊚ Ex. 6 2500.29 0.000 1.8 ⊚ Comp. Ex. 6 120 0.03 0.000 13.0 Δ2 Comp. Ex. 7 100 0.030.000 13.0 Δ2 Comp. Ex. 8 500 0.60 0.000 0.6 ×3 Comp. Ex. 9 550 0.620.000 0.5 ×3 Ex. 7 180 0.28 0.000 2.0 ⊚ Ex. 8 200 0.27 0.000 2.2 ⊚ Ex. 9210 0.28 0.000 1.6 ⊚ Ex. 10 220 0.27 0.000 0.5 ⊚ Ex. 11 200 0.27 0.0000.5 ⊚ Comp. Ex. 10 100 0.03 0.000 15.0 Δ2 Comp. Ex. 11 90 0.03 0.00015.0 Δ2 Comp. Ex. 12 580 0.64 0.000 0.5 ×3 Comp. Ex. 13 600 0.62 0.0000.4 ×3 Ex. 12 80 0.00 0.002 3.8 ⊚ Ex. 13 90 0.00 0.002 4.0 ⊚ Ex. 14 1000.00 0.006 1.9 ⊚ Ex. 15 180 0.00 0.060 0.4 ⊚ Ex. 16 200 0.00 0.050 0.5 ⊚Comp. Ex. 14 620 0.00 0.180 0.2 ×3 Ex. 17 200 0.00 0.050 1.9 ⊚ Ex. 18220 0.00 0.050 1.1 ⊚ Ex. 19 250 0.00 0.060 0.3 ⊚ Ex. 20 230 0.00 0.0500.4 ⊚ Ex. 21 240 0.00 0.050 2.2 ⊚ Ex. 22 250 0.00 0.050 1.5 ⊚ Ex. 23 2700.00 0.060 0.4 ⊚ Ex. 24 280 0.00 0.060 0.4 ⊚

[0380] As can be understood from Table 1, the photoreceptors and imageforming apparatus of the present invention can produce images havinggood image qualities with little abrasion even when used for a longtime. On the contrary, the comparative photoreceptor and image formingapparatus have at least one of the drawbacks of large abrasion andproducing undesired images such as black streaks, backgrounddevelopment, and tailing. Therefore, the comparative photoreceptors andimage forming apparatus are apparently inferior to the photoreceptorsand image forming apparatus of the present invention.

[0381] This document claims priority and contains subject matter relatedto Japanese Patent Applications Nos. 2000-057342 and 2001-018537, filedon Mar. 2, 2000 and Jan. 26, 2001, respectively, incorporated herein byreference.

[0382] Having now fully described the invention, it will be apparent toone of ordinary skill in the art that many changes and modifications canbe made thereto without departing from the spirit and scope of theinvention as set forth therein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. An electrophotographic photoreceptor comprisinga photosensitive layer on an electroconductive substrate, whereinnitrate ion is present on a surface of the photosensitive layer in anamount of from 50 to 300 μg/m².
 2. The electrophotographic photoreceptoraccording to claim 1, wherein a material comprising a fluorine atom anda carbon atom is further present on the surface of the photosensitivelayer.
 3. The electrophotographic photoreceptor according to claim 2,wherein a ratio of the number of fluorine atoms to the number of carbonatoms at the surface of the photosensitive layer is from 0.05 to 0.5. 4.The electrophotographic photoreceptor according to claim 2, wherein thematerial comprises polytetrafluoroethylene.
 5. The electrophotographicphotoreceptor according to claim 1, wherein a fatty acid metal salt isfurther present on the surface of the photosensitive layer.
 6. Theelectrophotographic photoreceptor according to claim 5, wherein thefatty acid metal salt comprises a zinc atom.
 7. The electrophotographicphotoreceptor according to claim 6, wherein the fatty acid metal salt iszinc stearate.
 8. The electrophotographic photoreceptor according toclaim 6, wherein a ratio of the number of zinc atoms to the number ofcarbon atoms at the surface of the photosensitive layer is from 0.001 to0.1.
 9. The electrophotographic photoreceptor according to claim 1,wherein the photoreceptor further comprises a protective layer as asurface layer, and wherein the protective layer comprises a resin. 10.The electrophotographic photoreceptor according to claim 9, wherein theprotective layer further comprises a filler.
 11. The electrophotographicphotoreceptor according to claim 9, wherein the protective layer furthercomprises a charge transport material.
 12. An image forming apparatuscomprising: an electrophotographic photoreceptor; a charger configuredto charge the electrophotographic photoreceptor; a light irradiatorconfigured to irradiate the photoreceptor with light to form anelectrostatic latent image on the electrophotographic photoreceptor; animage developer configured to develop the electrostatic latent imagewith a developer comprising a toner to form a toner image on theelectrophotographic photoreceptor a transfer configured to transfer thetoner image onto a receiving material; and a fixer configured to fix thetoner image on the receiving material, wherein the electrophotographicphotoreceptor comprises a photosensitive layer on an electroconductivesubstrate, and wherein nitrate ion is present on a surface of thephotosensitive layer in an amount of from 50 to 300 μg/m².
 13. The imageforming apparatus according to claim 12, further comprising a lubricantapplicator configured to apply a lubricant on the surface of thephotosensitive layer.
 14. The image forming apparatus according to claim13, wherein the lubricant comprises a fluorine atom and a carbon atom,and wherein a ratio of the number of fluorine atoms to the number ofcarbon atoms at the surface of the photosensitive layer is from 0.05 to0.5.
 15. The image forming apparatus according to claim 13, wherein thelubricant comprises a fluorine-containing resin.
 16. The image formingapparatus according to claim 15, wherein the fluorine-containing resinis polytetrafluoroethylene.
 17. The image forming apparatus according toclaim 13, wherein the lubricant comprises a fatty acid metal salt. 18.The image forming apparatus according to claim 17, wherein the fattyacid metal salt comprises a zinc atom.
 19. The image forming apparatusaccording to claim 18, wherein the fatty acid metal salt is zincstearate.
 20. The image forming apparatus according to claim 18, whereina ratio of the number of zinc atoms to the number of carbon atoms at thesurface of the photosensitive layer is from 0.001 to 0.1.
 21. The imageforming apparatus according to claim 12, wherein the electrophotographicphotoreceptor further comprises a protective layer as a surface layer ofthe photoreceptor, and wherein the protective layer comprises a resin.22. The image forming apparatus according to claim 21, wherein theprotective layer further comprises a filler.
 23. The image formingapparatus according to claim 21, wherein the protective layer furthercomprises a charge transport material.
 24. The image forming apparatusaccording to claim 12, wherein the developer further comprises alubricant.
 25. The image forming apparatus according to claim 24,further comprising a container comprising a replenishing toner and thelubricant included in the developer.
 26. The image forming apparatusaccording to claim 24, wherein the lubricant included in the developercomprises zinc stearate.
 27. The image forming apparatus according toclaim 12, wherein the light irradiator irradiates a light beam which hasa diameter not greater than 50 μm and which is modulated by imageinformation.
 28. The image forming apparatus according to claim 12,wherein the charger comprises one of a contact charger and a short rangecharger.
 29. The image forming apparatus according to claim 28, whereinthe charger charges the photoreceptor while applying a DC voltage whichis overlapped with an AC voltage.
 30. A process cartridge for an imageforming apparatus, comprising: a housing; and an electrophotographicphotoreceptor contained in the housing, wherein the electrophotographicphotoreceptor comprises a photosensitive layer on an electroconductivesubstrate, and wherein nitrate ion is present on a surface of thephotosensitive layer in an amount of from 50 to 300 μg/m².
 31. Theprocess cartridge according to claim 30, wherein a material comprising afluorine atom and a carbon atom is present on the surface of thephotosensitive layer, and wherein a ratio of the number of fluorineatoms to the number of carbon atoms at the surface of the photosensitivelayer is from 0.05 to 0.5.
 32. The process cartridge according to claim31, wherein the material comprises a fluorine-containing resin.
 33. Theprocess cartridge according to claim 32, wherein the fluorine-containingresin is polytetrafluoroethylene.
 34. The process cartridge according toclaim 30, wherein a fatty acid metal salt is further present on thesurface of the photosensitive layer.
 35. The process cartridge accordingto claim 34, wherein the fatty acid metal salt comprises a zinc atom.36. The process cartridge according to claim 35, wherein the fatty acidmetal salt is zinc stearate.
 37. The process cartridge according toclaim 34, wherein a ratio of the number of zinc atoms to the number ofcarbon atoms at the surface of the photosensitive layer is from 0.001 to0.1.
 38. The process cartridge according to claim 30, wherein theelectrophotographic photoreceptor further comprises a protective layeras a surface layer, and wherein the protective layer comprises a resin.39. The process cartridge according to claim 38, wherein the protectivelayer further comprises a filler.
 40. The process cartridge according toclaim 38, wherein the protective layer further comprises a chargetransport material.